There are numerous lighting applications requiring a high degree of service reliability. Traffic signals, navigational aids, emergency exit lighting, vehicle headlamps, display lamps, security lighting, and certain outdoor lighting, such as street or runway lamps, are examples of applications where failure of a lamp to perform its intended function may be hazardous to those who depend on the lamp or its associated equipment for information and/or illumination.
Incandescent lamps are frequently employed in highly reliable service applications because of their simplicity, versatility, and low cost. One way to insure service reliability is to replace each lamp with a new lamp before the filament of the original lamp has failed. The design life of a filament is a statistical characteristic so that lamp replacement necessarily must occur significantly before the mean or expected filament life in order to insure service to a high degree. Since the cost of replacement may exceed the cost of the lamp, this maintenance policy is expensive even if there is a market for the partially expired lamp.
Incandescent lamps having two or more filaments such that the second filament is automatically brought into service (or into full service) upon failure of the first filament are known in the prior art. The concept of providing a second or back-up filament is sound providing there is a cost-effective way of detecting when the first filament has failed so that lamp replacement may be made before the backup filament also fails. As will be seen in the following examples, means for detecting failure of the first filament in the prior art are lacking in the sense that detection is both costly and inconvenient, typically requiring on-site visual inspection of the lamp in operation.
In U.S. Pat. No. 2,161,443, issued on June 6, 1939, to Warshawsky, there is disclosed an incandescent lamp having multiple filaments such that upon burnout or destruction of one of the filaments, a reserve filament is automatically brought into operation, thus allowing further use of the lamp. The basic notion is to extend the life of the lamp by means of one or more reserve filaments rather than to maintain reliability.
In U.S. Pat. No, 1,859,661, issued May 24, 1932, to Falge, there is disclosed a lamp having an auxiliary filament primarily intended for an automobile headlamp application. In the event of burnout or breakage of the primary filament, the auxiliary filament is automatically illuminated with brightness sufficient to serve as a visual marker so that the width and position of the vehicle can be accurately determined by an observer. The auxiliary filament provides less illumination than the primary filament in order that failure of the primary filament may be ascertained by visual inspection.
There are various lamps of the prior art which are specially designed to be electrically connected in series with lamps of the same type. In the event of a filament failure in one lamp, the electrical current continues to flow through an alternate path in the affected lamp so that the remaining lamps of the series arrangement continue to operate. Various methods are known for activating the alternate path upon the failure of the primary filament, including employment of a secondary filament in the alternate circuit. See U.S. Pat. No. 1,713,752, issued May 21, 1929, to Eckhardt et al., in which the primary filament, when intact, acts as a shunt diverting most of the current away from the secondary filament.
In U.S. Pat. No. 1,717,283, issued June 11, 1929, to Van Horn et al., and U.S. Pat. No. 1,581,690, issued Apr. 20, 1926, to Powell, there are shown lamps having secondary filaments which are electrically isolated from the primary filament and its circuit until the primary filament fails whereupon arcing between the primary and secondary lead-in wires occurs and fuses the secondary filament into the primary circuit. In both patents, the preferred secondary filament has a lower luminous output than that of the primary filament so that failure of the primary filament may be readily ascertained by visual inspection. A somewhat degraded performance of the secondary filament is acceptable so that visual detection may be possible.
See also U.S. Pat. Nos. 2,084,176; 2,074,246; and 2,029,211; issued to Adler, Jr., issued on June 15, 1937; Mar. 16, 1937; and Jan. 28, 1936, respectively, and U.S. Pat. No. 3,319,115, issued May 9, 1967, to Smith. These patents disclose two-filament lamps in which both filaments are operated simultaneously such that failure of one of the filaments may be detected by visual inspection. In some cases, the light output of the partially failed lamp is noticeably different; in other cases, the light output may be the same but the appearance of the partially operational lamp is noticeably different (as when the positions of the operating filament or filaments within the lamp envelope are visible) so that visual detection is possible.
It would be a substantial advancement of the art if a two-filament incandescent lamp were provided with effective means other than visual inspection for detecting failure of the primary filament. In such a lamp, the luminous output of the secondary filament may be roughly equivalent to that of the primary filament so that there is no degradation in service after failure of the primary filament. A lamp with these capabilities will enhance safety and reliability as well as reduce maintenance costs.